1. Field of the Invention
The present invention relates to an improved process for converting bromide (e.g., hydrogen bromide) to bromine using hydrogen peroxide as an oxidant.
2. Description of the Prior Art
In the manufacture of aromatic bromine compounds from an aromatic precursor compound and bromine, about one-half of the bromine value is converted to hydrogen bromide by-product. In order to achieve an economically viable process, the hydrogen bromide has to be oxidized back to bromine. One conventional oxidizing agent which can be used is chlorine. However, use of chlorine has several disadvantages. One major disadvantage is the contamination of the bromine which is produced with chlorine, necessitating use of a purification step before the bromine can be used. Another disadvantage involves the need to dispose of the hydrogen chloride by-product which is formed. The use of hydrogen peroxide as the oxidizing agent would yield water as the by-product and would thus present no purification or disposal problem.
Hydrogen peroxide, however, can act as both an oxidizing and a reducing agent. Unless special techniques are used, it is not possible to obtain high conversions of bromine from hydrogen bromide using hydrogen peroxide as an oxidizing agent.
One limiting case is the oxidation of by-product hydrogen bromide with hydrogen peroxide in the presence of a very reactive aromatic substrate, often a phenol. In this case, as exemplified with the process of Japanese Patent Publication No. 54 39,044, as abstracted in Chem. Abstr. Vol. 91, 91363p, the bromine formed from the oxidation of hydrogen peroxide reacts so fast with the aromatic substrate that no significant amount of bromine is reduced back to hydrogen bromide. However, this is not a general process and would not lend itself, for example, to the polybromination of diphenyl ether.
German Offen. No. 2,713,345 (Chem. Abstr. Vol. 90, 57290g) mentions the addition of aqueous hydrogen peroxide to an aqueous bromide solution, followed by raising of the temperature from room temperature to 35.degree.-45.degree. C. followed by addition of an aqueous alkali bromate. Pure bromine is then distilled from the mixture. The use of sodium bromate, a second oxidizing agent, in such a procedure precludes a full liberation of the bromine values because the sodium bromate is reduced to sodium bromide.
U.S. Pat. No. 4,029,732 solves the problem of removing bromine when hydrogen peroxide is used as an oxidant from bromide ion by physically removing the bromine from the system as rapidly as it is formed by distillation. In the process shown in that patent, the hydrogen bromide can be derived from a bromosulfuric acid solution obtained from the fixation of bromine vapors with sulfur dioxide. Such solutions contain about 0.5 mole of sulfuric acid per mole of hydrogen bromide as a consequence of stoichiometry. This patent, however, stresses (at Col. 1, lines 61-66) that it is necessary to use equipment adapted to effect the separation of bromine substantially as rapidly as it is formed and in effect prevent the bromine from coming into contact with hydrogen peroxide. No mention is made of any favorable result achievable by use of strong acids even though this patent mentions the possible use of a bromosulfuric acid solution reaction medium in one embodiment which is said (at Col. 3, lines 3-7) to contain certain quantities of hydrobromic acid, sulfuric acid, and hydrochloric acid.
More recently, U.S. Pat. No. 4,447,405 teaches that bromide can be removed from arsenic acid solutions by treating such solutions with an oxidant selected from the group consisting of hydrogen peroxide, ozone, or chromium (VI). This patent recognizes that bromine can catalyze the decomposition of hydrogen peroxide and can itself be reduced to bromide by hydrogen peroxide and suggests that the removal efficiency can be improved by preventing the buildup of bromine by the controlled addition of hydrogen peroxide with the simultaneous removal of bromine. It does not suggest that use of a strong acid can improve the bromine conversion percentage in the oxidation of bromide by hydrogen peroxide.